Control apparatus and control method for optical modulator

ABSTRACT

An object of the invention is to provide a control system in which the phase shift between drive signals of an optical modulator can be reliably detected and compensated by a simple configuration. To this end, a control apparatus of the invention, for an optical modulator generating a signal light of a CS-RZ modulation system or the like by two LN modulators connected in series, detects the phase shift between drive signals given to the former and latter stage LN modulators, or judges the phase shift between the drive signals based on intensity information of the electric spectrum of the signal light output from the optical modulator, to control the phases of the drive signals so as to minimize the phase shift. As a result, the phase shift between the drive signals can be reliably detected and compensated by an electric circuit with a simple configuration.

This application is a divisional application of U.S. patent applicationSer. No. 10/793,097, filed Mar. 5, 2004, now U.S. Pat. No. 7,418,211 thedisclosure of which is herein incorporated in its entirety by reference,which claims the priority benefit of Japanese Application No.2003-088666, filed Mar. 27, 2003, the disclosure of which is hereinincorporated in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a control technique for an opticalmodulator used in an optical communication, and particularly, relates toa control technique for compensating for the phase shift between aplurality of drive signals driving the optical modulator and theoperating point deviation of the optical modulator.

2. Description of the Related Art

At present, a practical use of optical transmission system in which atransmission speed of optical signals is 10 Gb/s or the like, has beestarted. However, due to a recent rapid increase of network utilization,the larger network capacity has been required, and a demand for ultralong distance has been increased.

In an optical transmission system in which a transmission speed is equalto or more than 10 Gb/s, the wavelength dispersion significantly affectswaveforms, leading to a wider optical spectrum. As a result, a WDMtransmission in which channel lights are arranged in high densitybecomes difficult Particularly, in an optical transmission system of 40Gb/s, the wavelength dispersion is one of factors limiting atransmission distance.

As one means for solving the above described problems, a dispersioncompensation technique for accurately measuring a dispersion value of anoptical transmission path to compensate for the dispersion value hasbeen studied (refer to Japanese Unexamined Patent Publication No.11-72761 and Japanese Unexamined Patent Publication No. 2002-077053).Moreover, for realizing the above described optical transmission system,it is essential to develop a modulation system in which the dispersiontolerance is as large as possible. Specifically, a modulation system isrequired, in which an excellent optical signal to noise ratio can besecured for a long distance optical transmission system, that is to say,a modulation system is required, which is strong in the self phasemodulation (SPM) effect and can increase an upper limit of optical inputpower into the optical transmission path. Furthermore, a modulationsystem is required, in which the optical spectrum is narrow, to enable ahigh density WDM optical transmission for the large capacity.

Recently, as new modulation systems, a Carrier-Suppressed Return-to-Zero(hereunder, CS-RZ) modulation system and the like have been studied(refer to Y. Miyamoto et. al., “320 Gbit/s (8×40 Gbit/s) WDMtransmission over 367-km zero-dispersion-flattened line with 120-kmrepeater spacing using carrier-suppressed return-to-zero pulse format”,OAA′ 99 PD, PdP4). Since this CS-RZ modulation system has an advantagethat, as described later, the optical spectrum width becomes ⅔ timescompared to the Return-to-Zero (RZ) modulation system, the wavelengthdispersion tolerance is large, which enables a high density channellight arrangement in the WDM. Furthermore, since the waveformdeterioration due to the self phase modulation effect is small, itbecomes possible to secure the optical signal to noise ratio for thelong distance transmission.

FIG. 25 is a diagram showing a basic configuration for generating aCS-RZ modulating signal of 40 Gb/s.

In FIG. 25, a light source 100 generates a continuous light Thecontinuous light output from the light source 100 is sequentially inputto two LiNbO₃ modulators (hereunder, LN modulators) 110 and 120connected in series, to be modulated.

The former stage LN modulator 110 is applied with, at a signal electrodethereof (not shown in the figure), for example, a data signal with bitrate of 40 Gb/s, which is generated in a data signal generating section111 and corresponds to the NRZ modulation system, as a drive signal. Asa result, the former stage LN modulator 110 modulates the continuouslight from the light source 100 in accordance with the data signal, andoutputs an NRZ signal light of 40 Gb/s having a waveform as exemplifiedin (a) of FIG. 26 to the latter stage LN modulator 120.

For the latter stage LN modulator 120, for example, a Mach-Zehnder (MZ)modulator or the like having two signal electrodes is used. The latterstage LN modulator 120 is applied with, at the respective signalelectrodes thereof, a first drive signal and a second drive signalgenerated based on a clock signal having a frequency of ½ the bit rateof the data signal. As a result, the latter stage LN modulator furthermodulates the NRZ signal light from the former stage LN modulator 110,and outputs a CS-RZ signal light of 40 Gb/s having a waveform asexemplified in (b) of FIG. 26. Here, a clock signal having a waveform ofa sine wave and the like with frequency 20 GHz, is generated in a clocksignal generating section 121. The clock signal is branched into two bya branching device 124, and then adjusted by phase shifters 125A and125B so that a phase difference between branched signals reachesapproximately 180°. Furthermore, respective amplitudes of the branchedsignals are adjusted by amplifiers 126A and 126B, to become first andsecond drive signals to be applied to the respective signal electrodesof the LN modulator 120.

Moreover, a part of the clock signal generated in the clock signalgenerating section 121 is branched by a branching device 122 andtransmitted to the data signal generating section 111 so that phases ofthe data signal and clock signal are synchronized, and at the same time,the phase of the clock signal is adjusted by a phase shifter 123 so thata phase difference between the respective signals is controlled.

Here, the theory of how the CS-RZ signal light of 40 Gb/s is generatedis briefly described using an optical intensity characteristic of an LNmodulator to a drive voltage, shown in FIG. 27.

Generally, in the case where a signal light corresponding to the NRZmodulation system or the RZ modulation system is generated using anoptical modulator, an optical intensity characteristic of which to adrive voltage is changed periodically, a drive voltage corresponding toadjacent “top, bottom” or “bottom, top” of the above optical intensitycharacteristic (hereunder, this drive voltage is Vπ) is given to theoptical modulator, so as to perform the modulation. Here, “top” of theoptical intensity characteristic denotes emission peak points and“bottom” denotes extinction peak points.

On the other hand, in the case where a signal light corresponding to theCS-RZ modulation system is generated, the signal light of 40 Gb/smodulated by the former stage LN modulator 110 shown in FIG. 25 inaccordance with the data signal, is further modulated by the latterstage LN modulator 120 in accordance with the clock signal of 20 GHzhaving the frequency of ½ the bit rate of the data signal. The latterstage LN modulator 120, as shown in the left of FIG. 27, is applied witha drive voltage corresponding to “top, bottom, top” of the opticalintensity characteristic to the drive voltage (hereunder, this drivevoltage is 2Vπ). This light modulation is performed by corresponding therespective levels of −1, 0, 1 of the clock signal to the respectiveconditions of ON, OFF, ON of the light. As a result, the CS-RZ signallight generated becomes a binary optical waveform as shown at the topright of FIG. 27. For the signal light in this CS-RZ modulation system,since optical phases of respective bits thereof have a value of 0 or π,for example, as shown in a calculation result of the optical spectrum atthe bottom right of FIG. 27, a carrier component of the optical spectrumis suppressed.

For the signal light of the CS-RZ modulation system generated asdescribed above, for example, as in the respective experimental resultsof the optical spectrum and optical waveform shown in FIG. 28, anoptical waveform of the form approximately the same as the opticalwaveform of the RZ modulation system can be obtained, and the opticalspectrum width becomes narrower than that of the RZ modulation system.Moreover, as in the experimental results related to the wavelengthdispersion tolerance shown in FIG. 29, a range of total wavelengthdispersion where a value of power penalty becomes equal to or less than1 dB, is approximately 40 ps/nm in the RZ modulation system, whereas inthe CS-RZ modulation system, the range is approximately 50 ps/nm.Accordingly, it is understood that, for the signal light of the CS-RZmodulation system, the dispersion tolerance is enlarged compared to thesignal light of the RZ modulation system.

Incidentally, the signal light corresponding to the CS-RZ modulationsystem has the above described advantages, but there are disadvantagesin that; the phase between the first and second drive signals to begiven to the latter stage optical modulator which is driven based on theclock signal, should be precisely adjusted, and the phase between theabove described clock signal and the data signal used for driving theformer stage optical modulator should also be precisely adjusted.Furthermore, since there is a possibility that the phase shift occursdue to environmental changes such as temperature changes, it becomesessential to detect a phase change in each signal during the systemoperation, to perform a feedback control.

Here, the present applicant has proposed a system, for example as shownin FIG. 30, for monitoring the optical spectrum of signal light outputfrom an optical modulator by a monitoring section 130, and then based onan intensity variation of a specific frequency component in the opticalspectrum, feedback controlling the above described phase shift betweendrive signals by a control circuit 140 (refer to Japanese PatentApplication No. 2002-087017). According to this prior invention, byfocusing on the intensity variation of the specific frequency componentof the output optical spectrum, the phase shift between signals of thedrive system can be reliably detected, and it becomes possible tocontrol the phase difference between drive signals so that an optimumdrive condition can be obtained stably.

However, the control system of the optical modulator according to thisprior invention has the following problems. That is, in the abovedescribed control method, the specific frequency component of the outputoptical spectrum is extracted using a narrow-band optical filter 132, tomonitor the intensity variation. However, at this time, there is aproblem that, unless the specific frequency component is extracted usingan optical filter with a sufficiently narrow bandwidth of transmissionband, the monitoring accuracy of the intensity variation is reduced.Generally, an optical filter having a sufficiently narrow bandwidth isnot easily realized. Therefore, due to the reduction of monitoringaccuracy of the intensity variation as described above, there is apossibility that it becomes difficult to feedback control stably thephase difference between the drive signals. Furthermore, for the controlsystem of the prior invention, another problem is that a controlcorresponding to the operating point variation of the optical modulatorhas not yet been realized.

SUMMARY OF THE INVENTION

The present invention has been accomplished in view of the aboveproblems, and has a first object to provide a control system capable ofreliably detecting the phase shift between drive signals of an opticalmodulator to compensate for the deviation with a simple configuration.Moreover, the present invention has a second object to provide a controlsystem capable of compensating for the operating point deviation of anoptical modulator driven with the voltage amplitude of 2Vπ.

An optical modulator which is an object to be controlled by a controlapparatus according to the present invention for achieving the abovefirst object, comprises: a first modulation section and a secondmodulation section connected in series; and a drive section that givesdrive signals, phases of which are synchronized, to the first and secondmodulation sections, respectively. The second modulation sectionincluding a part for branching an optical waveguide into a first branchoptical waveguide and a second branch optical waveguide, and a part forcombining the first and second branch optical waveguides, utilizes afirst electrode and a second electrode respectively provided in thefirst and second branch optical waveguides to control refractive indexesof the first and second branch optical waveguides, and obtains aperiodic optical intensity characteristic according to a differencebetween the refractive indexes. Moreover, the drive section is capableof giving a drive signal to at least one of the first and secondelectrodes so that the second modulation section performs a modulatingoperation corresponding to one period of the optical intensitycharacteristic thereof. For such an optical modulator, one aspect of thepresent control apparatus comprises: a phase shift detection sectionthat compares phases of the respective drive signals given to the firstand second modulation sections to detect the phase shift, and a controlsection that controls the drive section so as to minimize the phaseshift detected in the phase shift detection section. In such aconstitution, the phase shift between respective drive signals given tothe first and second modulators from the drive section, is detected inthe phase shift detection section, and based on the detection result,the drive section is controlled so that the phases of the respectivedrive signals are adjusted. As a result, it becomes possible to detectthe phase shift between the drive signals given to the first and secondmodulators, to compensate for the phase shift by an electric circuitwith a simple configuration.

Furthermore, another aspect of the control apparatus for the opticalmodulator comprises: an output monitoring section that photo-electricconverts a signal light output from the optical modulator to acquire theelectric spectrum, and detects information related to the intensity ofthe electric spectrum; and a control section that judges the phase shiftbetween the drive signals of the optical modulator based on theintensity information detected in the output monitoring section, andcontrols the drive section so as to minimize the phase shift. In such aconstitution, the phase shift between the drive signals is judged basedon the intensity information of the electric spectrum of the outputlight from the optical modulator, and based on the judgment result, thedrive section is controlled so that the phases of the drive signals areadjusted. As a result, it becomes possible to detect the phase shiftbetween the respective drive signals given to the optical modulator, tocompensate for the phase shift by an electric circuit with a simpleconfiguration.

The optical modulator which is an object to be controlled by the controlapparatus according to the present invention for achieving the abovesecond object comprises: a modulation section including a part forbranching an optical waveguide into a first branch optical waveguide anda second branch optical waveguide, and a part for combining the firstand second branch optical waveguides, and having a constitution ofutilizing a first electrode and a second electrode respectively providedin the first and second branch optical waveguides to control refractiveindexes of the first and second branch optical waveguides, and obtaininga periodic optical intensity characteristic according to a differencebetween the refractive indexes; a drive section that gives a drivesignal to at least one of the first and second electrodes so that themodulation section performs a modulating operation corresponding to oneperiod of the optical intensity characteristic thereof; and a biassupply section that supplies a DC bias to the modulation section toadjust an operating point. For such an optical modulator, the presentcontrol apparatus comprises: an output monitoring section that detects achange in the signal light output from the optical modulation section;and a control section that judges the operating point deviation of themodulation section based on the detection result in the outputmonitoring section, and controls the bias supply section so as tominimize the operating point deviation. In such a constitution, theoperating point deviation is judged based on the change in the outputlight from the optical modulator, and based on the judgment result, thebias supply section is controlled so that the DC bias given to themodulation section is adjusted. As a result it becomes possible tocompensate for the operating point deviation of the optical modulatordriven by voltage amplitude of 2Vπ.

Other objects, features and advantages of the present invention willbecome apparent from the following description of embodiments, inconjunction with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a first embodimentof the present invention.

FIG. 2 is a block diagram showing another configuration example relatedto the first embodiment.

FIG. 3 is a block diagram showing a further configuration examplerelated to the first embodiment.

FIG. 4 is a block diagram showing a configuration of a second embodimentof the present invention.

FIG. 5 is a diagram showing examples of electric spectrum of outputlight generated when a phase between a data signal and a clock signal ischanged.

FIG. 6 shows examples of optical waveform of output light generated whena phase between a data signal and a clock signal is changed.

FIG. 7 is a diagram for explaining a detection operation of phase shiftin the second embodiment.

FIG. 8 is a block diagram showing a configuration of a third embodimentof the present invention.

FIG. 9 is a diagram showing examples of electric spectrum of outputlight generated when a phase between dual system clock signals ischanged.

FIG. 10 is a diagram showing examples of optical waveform of outputlight generated when a phase between dual system clock signals ischanged.

FIG. 11 is a diagram for explaining a detection operation of phase shiftin the third embodiment.

FIG. 12 is a block diagram showing a schematic configuration of acontrol apparatus applied with a known operating point compensationsystem.

FIG. 13 is a diagram for explaining the theory of how the controlapparatus of FIG. 12 stabilizes the operating point.

FIG. 14 is a block diagram showing a configuration of a fourthembodiment of the present invention.

FIG. 15 is a diagram for explaining a change in output light when theoperating point deviation occurs in a modulator driven with voltageamplitude of 2Vπ.

FIG. 16 is a diagram for explaining a detection operation of operatingpoint deviation in the fourth embodiment.

FIG. 17 is a block diagram showing a configuration of a fifth embodimentof the present invention.

FIG. 18 is a diagram for explaining a detection operation of operatingpoint deviation in the fifth embodiment.

FIG. 19 is a block diagram showing another configuration example relatedto the fifth embodiment.

FIG. 20 is a block diagram showing an embodiment in which theconfigurations of the first, third and fourth embodiments are combined.

FIG. 21 is a block diagram showing an application example related to theembodiment of FIG. 19.

FIG. 22 is a block diagram showing an embodiment in which theconfigurations of the first, third and fifth embodiments are combined

FIG. 23 is a block diagram showing an embodiment in which theconfigurations of the second, third and fourth embodiments are combined.

FIG. 24 is a block diagram showing an embodiment in which theconfigurations of the second, third and fifth embodiments are combined.

FIG. 25 shows a basic configuration of a conventional CS-RZ modulationoptical modulator.

FIG. 26 shows examples of waveforms of signal light generated in thebasic configuration of FIG. 25.

FIG. 27 is a diagram for explaining the theory of how CS-RZ signal lightis generated.

FIG. 28 is a diagram showing experimental results for explainingcharacteristics related to the optical spectrum and optical waveform forCS-RZ signal light.

FIG. 29 is a diagram showing experimental results for explaining acharacteristic related to wavelength dispersion tolerance for CS-RZsignal light.

FIG. 30 is a block diagram showing a control apparatus for an opticalmodulator according to a prior invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereunder is a description of embodiments of the present invention basedon drawings. Throughout the drawings, the same reference numerals denotethe same or corresponding parts.

FIG. 1 is a block diagram showing a configuration of a first embodimentof a control apparatus for an optical modulator according to the presentinvention.

In FIG. 1, an optical modulator to which the control apparatus of thefirst embodiment is applied, sequentially inputs, for example, acontinuous light generated by a light source 1 to LN modulators 10 and20 connected in series serving as first and second optical modulators,to modulate the input light, and outputs a signal light of a CS-RZmodulation system. For this optical modulator, the present controlapparatus comprises: a phase comparator 30 serving as a phase shiftdetection section that detects the phase shift between a drive signalgiven to the former stage LN modulator 10 and a drive signal given tothe latter stage LN modulator 20; and a control circuit 31 serving as acontrol section that controls a phase shifter 23 so that the phase shiftdetected by the phase comparator 30 is minimized, and optimizes relativephases of the respective drive signals. Hereunder is a specificdescription of the components.

The former stage LN modulator 10 is a typical optical modulatorconfigured using a lithium niobate (LiNbO₃:LN) substrate. This formerstage LN modulator 10 is driven in accordance with, for example, a datasignal DATA (for example, a 40 Gb/s data signal) which is generated in adata signal generating section 11 and has a bit rate of B (b/s)corresponding to the NRZ modulation system, to modify the continuouslight from the light source 1 to output an NRZ signal light of B (b/s)to the latter stage LN modulator 20.

In the above data signal generating section 11, based on a data signalhaving a bit rate of B/n (b/s) corresponding to a plurality of (here, n)channels given from the outside, a data signal DATA of B/n (b/s)corresponding to the NRZ modulation system is generated, and the datasignal DATA is given to the LN modulator 10, and at the same time, here,a clock signal CLKd having a frequency of B/n (Hz) and generated byextracting a clock component from the data signal DATA of B/n (b/s), isoutput to the phase comparator 30.

The latter stage LN modulator 20 is a well-known Mach-Zehnder opticalmodulator configured using the lithium niobate substrate. This LNmodulator 20, includes, specifically; a part for branching an opticalwaveguide into a first optical waveguide and a second optical waveguide,and a part for combining the first and second optical waveguides, andutilizes a first electrode 20A and a second electrode 20B respectivelyprovided in the first and second optical waveguides, to controlrefractive indexes of the first and second optical waveguides so that aperiodic optical intensity characteristic corresponding to a differencebetween the refractive indexes is obtained. Here, an example is shownwhere the LN substrate is used for the former stage and latter stagemodulators. However, the substrate material is not limited thereto, andeach of the former stage and latter stage modulators may be configuredusing a substrate consisting of well-known material having anelectro-optic effect.

To the respective electrodes 20A and 20B of the LN modulator 20, clocksignals CLK1 and CLK2 (for example, 20 GHz clock signals), having afrequency corresponding to ½ the bit rate of the data signal generatedin the data signal generating section 11, that is, a frequency of B/2(Hz), are applied as drive signals. A phase difference between therespective clock signals CLK1 and CLK2 of B/2 (Hz) is adjusted by aphase shifter 25 and also the amplitudes of the respective clock signalsare adjusted by amplifiers 26A and 26B, so that a potential differencebetween the respective electrodes 20A and 20B for when the clock signalsCLK1 and CLK2 are given to the LN modulator 20, corresponds to oneperiod of the periodic optical intensity characteristic of the LNmodulator 20 (refer to FIG. 27). The latter stage LN modulator 20 drivenin accordance with such respective clock signals CLK1 and CLK2, furthermodulates the NRZ signal light from the former stage LN modulator 10, tooutput a CS-RZ signal light of B (b/s).

A clock signal generating section 21 generates, for example, a clocksignal CLK0 of B/2 (Hz) having a waveform of a sine wave or the like.This clock signal CLK0 of B/2 (Hz) is branched into two by a branchingdevice 22 to be sent respectively to the data signal generating section11 and the phase shifter 23. The signal sent from the branching device22 to the data signal generating section 11 is used as a synchronoussignal of the data signal DATA of B (b/s) generated by the data signalgenerating section 11.

The phase shifter 23, as described later, adjusts a phase of the clocksignal CLK0 sent from the branching device 22 in accordance with acontrol signal S_(FB) output from the control circuit 31. As the phaseshifter 23, for example, it is possible to use a variable-length coaxialtube, a voltage controlled device or the like. The clock signal CLK0 ofB/2 (Hz) that has been phase adjusted by the phase shifter 23, isbranched into three clock signals CLK1, CLK2 and CLK3 by a branchingdevice 24, to be sent respectively to the phase shifter 25, theamplifier 26B and the phase comparator 30.

The phase shifter 25, here, adjusts a phase of the clock signal CLK1sent from the branching device 24, in order to adjust the phasedifference between the drive signals respectively given to theelectrodes 20A and 20B of the latter stage LN modulator 20. As the phaseshifter 25, for example, it is also possible to use a variable-lengthcoaxial tube, a voltage controlled device or the like. The clock signalCLK1 that has been phase adjusted by the phase shifter 25, is given tothe first electrode 20A of the LN modulator 20 after its amplitude isadjusted to a required level by the amplifier 26A. On the other hand,the clock signal CLK2 that has been branched by the branching device 24is here sent to the amplifier 26B without passing through any phaseshifter, to be given to the second electrode 20B of the LN modulator 20after its amplitude is adjusted to a required level by the amplifier26B.

Here, the phase shifter 25 is provided only on the clock signal CLK1side. However, also on the clock signal CLK2 side, a phase shifter mayalso be provided between the branching device 24 and the amplifier 26B,so that the phase adjustment is performed on both the clock signals CLK1and CLK2.

The phase comparator 30 compares between a phase of a clock signal CLKdof B/2 (Hz) output from the data generating section 11 and a phase ofthe clock signal CLK3 of B/2 (Hz) branched by the branching device 24,and detects the phase shift between the respective clock signals CLKdand CLK3, to output a signal indicating the detection result to thecontrol circuit 31. This phase comparator 30 is arranged so that aphysical length L_(CLK3) of a signal line which propagates the clocksignal CLK3 between itself and the branching device 24 becomes the sameas respective physical lengths L_(CLK1) and L_(CLK2) of signal lineswhich respectively propagates the clock signal CLK1 between thebranching device 24 and the electrode 20A, and the clock signal CLK2between the branching device 24 and the electrode 20B, of the LNmodulator 20 (L_(CLK1)=L_(CLK2)=L_(CLK3)). In this manner, the physicallengths L_(CLK1) to L_(CLK3) of the respective signal lines are set tobe the same. Consequently, even if the physical lengths L_(CLK1) toL_(CLK3) of the respective signal lines are changed due to a temperaturechange or the like, there is no difference between changes in therespective clock signals. As a result, it becomes possible to compare,with high accuracy, phases between the data signal and the clock signal,even though the clock signal CLK3 for monitoring is used.

The control circuit 31 generates a control signal S_(FB), according toan output signal from the phase comparator 30, for feedback controllinga phase adjustment amount in the phase shifter 23 so that the phaseshift between the clock signals CLKd and CLK3 is minimized.

In the optical modulator to which the control apparatus of the aboveconfiguration is applied, the continuous light from the light source 1is input to the former stage LN modulator 10. The data signal DATA of B(b/s) generated by the data signal generating section 11 has been givento the LN modulator 10 as the drive signal. The continuous light inputto the LN modulator 10 is modulated in accordance with the data signalDATA to become the NRZ signal light of B (b/s), and is output from theformer stage LN modulator 10, to be sent to the latter stage LNmodulator 20.

In the latter stage LN modulator 20, the clock signals CLK1 and CLK2obtained by phase adjusting the clock signal CLK0 of B/2 (Hz) generatedby the clock signal generating section 21 by the phase shifter 23 andthen branching the phase adjusted clock signal CLK0 into the clocksignals CLK1 and CLK2 by the branching device 24, and further adjustingthe phases and amplitudes thereof by the phase shifter 25, and theamplifiers 26A and 26B, have been respectively given to the first andthe second electrodes 20A and 20B as the drive signals. At this time,phase adjustment on the clock signal CLK0 in the phase shifter 23 isfeedback controlled in accordance with the control signal S_(FB) outputfrom the control circuit 31.

In this feedback control, specifically, the phase adjustment amount inthe phase shifter 23 is optimized so that the phase shift detected bythe phase comparator 30, that is, the phase shift between the clocksignal CLKd of B/2 (Hz) extracted from the data signal DATA of B (b/s)driving the former stage LN modulator 10 and the clock signal CLK3equivalent to the clock signals CLK1 and CLK2 of B/2 (Hz) driving thelatter stage LN modulator 20, is minimized and finally becomesapproximately zero, to automatically compensate for the phase shiftbetween the data signal DATA, and the clock signals CLK1 and CLK2.

For the clock signals CLK1 and CLK2 of B/2 (Hz) in which the phase shiftbetween the data signal DATA of B (b/s) is adjusted by the abovefeedback control, the phase difference therebetween is further adjustedby the phase shifter 25 so that the potential difference between therespective electrodes 20A and 20B for when the clock signals CLK1 andCLK2 are sent to the latter stage LN modulator 20 corresponds to oneperiod of the periodic optical intensity characteristic of the LNmodulator 20, and the amplitudes thereof are adjusted by the amplifiers26A and 26B. In the LN modulator 20 where the clock signals CLK1 andCLK2 adjusted in these manner are given to the respective electrodes asthe drive signals, the NRZ signal light from the former stage LNmodulator 10 is modulated in accordance with the clock signal of B/2(Hz), and the CS-RZ signal light of B (b/s) in which the waveformdeterioration due to the phase shift between the data signal DATA, andthe clock signals CLK1 and CLK2 has been suppressed, is output to theoutside.

As a specific operation mode of such an optical modulator, for example,at the time of system introduction, the phase adjustment amounts of therespective phase shifters 23 and 25 are set to values at which outputwaveforms are optimized by manual operation or the like. Then, in thisstate, the feedback control by the phase comparator 30 and the controlcircuit 31 is started. Thus, since the phase shift between the datasignal DATA, and the clock signals CLK1 and CLK2, occurring due to thetemperature change or the like when the system is operated, is reliablydetected, it becomes possible to automatically compensate for the phaseshift.

Thus, according to the control apparatus for the optical modulator ofthe first embodiment, the phase shift between the clock signal CLKd ofB/n (Hz) extracted from the data signal DATA of B/n (b/s) driving theformer stage LN modulator 10 and the clock signal CLK3 equivalent to theclock signals CLK1 and CLK2 of B/2 (Hz) driving the latter stage LNmodulator 20, is detected, and then based on the detection result, thephase shifter 23 is feedback controlled. Consequently, differently fromthe above described prior invention, without monitoring the opticalspectrum of the signal light output from the latter stage LN modulator20, it becomes possible to reliably and automatically compensate for thephase shift between the data signal DATA, and the clock signals CLK1 andCLK2 by only an electric circuit with a simple configuration.

In the above first embodiment, the description has been made for theoptical modulator in which the former stage LN modulator 10 is driven bythe data signal DATA of B/n (b/s) and the latter stage LN modulator 20is driven by the clock signals CLK1 and CLK2 of B/2 (Hz), to generatethe CS-RZ signal light of B (b/s). However, the present invention is notlimited thereto. For example, as shown in FIG. 2, similarly to the firstembodiment, it is also possible to apply the control apparatus of thepresent invention to a modulator or the like in which a clock signalCLK1′ having the frequency of B (Hz) corresponding to the bit rate ofthe data signal DATA driving the former stage LN modulator 10, is givento one electrode 20A of the latter stage LN modulator 20, while theother electrode 20B is earthed, to generate an RZ signal light of B(b/s). In this case, it is provided that the data signal generatingsection 11 outputs a clock signal CLKd′ of B/n (Hz) extracted from thedata signal DATA of B/n (b/s) to the phase comparator 30, and the phasecomparator 30 compares between a phase of the clock signal CLKd′ and aphase of a clock signal CLK2′ branched by a branching device 24. Herealso, a physical length L_(CLK1′) of a signal line which propagates theclock signal C_(LK1′) between the branching device 24 and the electrode20A of the latter stage LN modulator 20 and a physical length L_(CLK2′)of a signal line which propagates the clock signal CLK2′ between thebranching device 24 and the phase comparator 30 are set to become thesame (L_(CLK1′)=L_(CLK2′)). Furthermore, as a specific configuration ofthe phase comparator 30, for example, a D flip-flop may be used, and theconstitution may be such that the clock signal CLK2′ from the branchingdevice 24 and the clock signal CLKd′ from the data signal generatingsection 11 are respectively given to a data input terminal and a clockinput terminal of the D flip-flop, to generate the control signal S_(FB)by the control circuit 31 using an output signal from the D flip-flop.In addition, in the configuration of FIG. 2, if a frequency of the clocksignal CLK1′ is set to B/2 (Hz) and the latter stage LN modulator 20 isdriven with the voltage amplitude of 2×Vπ, it is also possible togenerate the CS-RZ signal light of B (b/s).

Furthermore, in the first embodiment, the description has been made onthe constitution in which the former stage LN modulator is driven by thedata signal and the latter stage LN modulator is driven by the clocksignal. However, as shown in FIG. 3, the constitution may be such thatthe positions of the former stage LN modulator and the latter stage LNmodulator are interchanged, so that the former stage LN modulator isdriven by the clock signal and the latter stage LN modulator is drivenby the data signal. Such a constitution may also be applied to the otherembodiments described hereunder.

Next is a description of a second embodiment of the control apparatusfor the optical modulator according to the present invention.

FIG. 4 is a block diagram showing a configuration of the controlapparatus for the optical modulator according to the second embodiment.

In FIG. 4, similarly to the first embodiment, the control apparatus ofthe second embodiment is applied to the optical modulator in which theformer stage LN modulator 10 is driven by the data signal DATA and thelatter stage LN modulator 20 is driven by the clock signals CLK1 andCLK2, to output the signal light of the CS-RZ modulation system. Theconstitution of the present control apparatus differs from that of thefirst embodiment in that, instead of the phase comparator 30 and thecontrol circuit 31 in the first embodiment, there are provided: anoutput monitoring section 40 that branches a part of the signal lightoutput from the latter stage LN modulator 20 as a monitor light, andthen photo-electric converts this to acquire the electric spectrum, andmonitors the intensity of a specific frequency component of the electricspectrum; and a control circuit 50 that, based on an intensity variationof the specific frequency component monitored by the output monitoringsection 40, judges the phase shift between the data signal DATA, and theclock signals CLK1 and CLK2, to feedback control the phase shifter 23.Since other components are similar to those of the first embodiment, thedescription thereof is omitted here.

The output monitoring section 40 includes, for example, an opticalcoupler 41, a light receiving circuit 42, an electric filter 43, and anelectric power sensor 44. The optical coupler 41 branches a part of theCS-RZ signal light output from the latter stage LN modulator 20 as amonitor light, to send this to the light receiving circuit 42. The lightreceiving circuit 42 is a circuit photo-electric converting the monitorlight branched by the optical coupler 41 to acquire the electricspectrum. The electric filter 43 is an electric band-pass filter capableof extracting a specific frequency component the intensity of which ischanged most largely corresponding to the phase shift between the datasignal and clock signals, from the electric spectrum obtained by thelight receiving circuit 42. The above specific frequency component willbe described later. The electric power sensor 44 measures the intensityof an electric signal extracted by the electric filter 43, to output themeasurement result to the control circuit 50.

The control circuit 50 generates the control signal S_(FB) for feedbackcontrolling the phase adjustment amount of the phase shifter 23 so thatthe intensity of the specific frequency component measured by theelectric power sensor 44 becomes a maximum. This feedback control by thecontrol circuit 50 is performed based on a characteristic of change inthe electric spectrum of the CS-RZ signal light to a phase changebetween the data signal DATA, and the clock signals CLK1 and CLK2 asdescribed in the next.

FIG. 5 shows examples of the electric spectrum of the CS-RZ signal lightgenerated when the phase between a data signal of 40 Gb/s and a clocksignal of 20 GHz is changed. Moreover, FIG. 6 shows examples of theoptical waveform of the CS-RZ signal light generated when the phase ischanged similarly to FIG. 5. Here, with a condition where the phasebetween the data signal and the clock signals is optimized (delay timedue to the phase shift between signals is 0 ps) as a reference, thephase of the clock signal is changed until the original optimum phasecondition (delay time is 25 ps) is restored after the phase shiftcontinues to be increased.

As shown in FIG. 6, even if the phase shift corresponds to only 5 ps (1mm if converted into coaxial cable length) of a delay time between thedata signal and the clock signals, it is understood that the opticalwaveform of the CS-RZ signal light is largely deteriorated. At thistime, as shown in FIG. 5, if the phase between the data signal, and theclock signals is shifted from an optimum point, it is understood thatthe intensity of the electric spectrum of the CS-RZ signal light in thedomain separated to the lower side from the 40 GHz frequencycorresponding to the bit rate of the data signal (in the example of FIG.5, the frequency domain spanning several GHz with a center ofapproximately 25 GHz) is reduced.

Therefore, in the present embodiment, paying attention to the specificfrequency component the intensity of which is largely changedcorresponding to the phase shift between the data signal and the clocksignals as described above, a generation condition of the phase shift isjudged based on the intensity change in the specific frequencycomponent, to feedback control the phase shifter 23 so that the phaseshift between the data signal and the clock signals is optimized.

Specifically, in the case where the electric spectrum corresponding tothe above described CS-RZ signal light of 40 Gb/s is obtained by thelight receiving circuit 42, then as shown by the broken line portion inFIG. 7, a central frequency of transmission band of the electric filter43 is set to match with a frequency of approximately 25 GHz at which theintensity is changed most largely corresponding to the phase shift. Asis also apparent from FIG. 7, the less the phase shift between the datasignal and the clock signals becomes, the more the intensity of thespecific frequency component extracted by this electric filter 43 isincreased. Therefore, the phase shifter 23 is feedback controlled sothat the intensity measured by the electric power sensor 44 becomes amaximum, thus it becomes possible to optimize the phase differencebetween the data signal and the clock signals.

Furthermore, since the above electric filter 43 has a characteristicsuch that the bandwidth of the transmission band thereof is as narrow aspossible and the transmissivity is sharply changed at both ends of thetransmission band, it becomes possible to detect the phase shift betweenthe data signal and the clock signals with higher accuracy. To realizean electric filter having such a sharp filter characteristic in thenarrow band is easy compared to the optical filter used in the priorinvention described above. Therefore, the phase difference between thedata signal and the clock signals can be more stably optimized.

Furthermore, in the control circuit 50, a maximum value of the intensitymeasured by the electric power sensor 44 is detected by applying awell-known processing such as the dithering. Thus, it is also possibleto detect a traveling direction of the phase shift. If in this manner,the traveling direction of the phase shift is detected to feedbackcontrol the phase shifter 23, the phase difference between the datasignal and the clock signals can be optimized at higher speed.

As described above, according to the second embodiment, the intensitychange in the specific frequency component for the electric spectrum ofthe output light from the latter stage LN modulator 20 is monitored.Thus, it becomes possible to reliably detect the phase shift between thedata signal and the clock signals to feedback control the phase shifter23. As a result, it becomes possible to generate the CS-RZ signal lightin a stable drive condition.

In the second embodiment, the description has been made on the casewhere a band-pass filter is used as the electric filter 43. However, inthe present invention, the electric filter extracting the specificfrequency component from the electric spectrum of the output light isnot limited to the above. For example, it is also possible to use alow-pass filter having the cut-off frequency in the lower domain thanthe frequency corresponding to the bit rate of the data signal and alsoin the higher domain than the frequency at which the intensity ischanged most largely according to the phase shift. However, in order todetect the phase shift with higher accuracy, it is desirable to use theband-pass filter.

Next is a description of a third embodiment of the control apparatus forthe optical modulator according to the present invention.

In the above described second embodiment, the case has been shown where,based on the electric spectrum of the output light, the phase shiftbetween the data signal and the clock signals is detected to feedbackcontrol the phase shifter 23. In the third embodiment, the descriptionis made on the control apparatus in which, based on the electricspectrum of the output light, the phase shift between the clock signalsCLK1 and CLK2 driving the latter stage LN modulator 20, is detected tofeedback control the phase shifter 23.

FIG. 8 is a block diagram showing a configuration of the controlapparatus for the optical modulator according to the third embodiment.

In FIG. 8, the constitution of the present embodiment differs from thatof the second embodiment in that there is provided an output monitoringsection 40′ with the electric filter 43 omitted from the outputmonitoring section 40 used in the second embodiment. A monitoring resultin this output monitoring section 40′ is sent to the control circuit 50,and the control circuit 50 feedback controls phase shifters 25A and 25Bso that a phase difference between the clock signals CLK1 and CLK2 isoptimized. Here, a configuration example is shown, in which the phaseshifter 25A is arranged between the branching device 24 and theamplifier 26A, and the phase shifter 25B is arranged between thebranching device 24 and the amplifier 26B corresponding to the clocksignals CLK1 and CLK2 which are given to the electrodes 20A and 20B ofthe latter stage LN modulator 20, to adjust the phase difference betweenthe clock signals CLK1 and CLK2 by the two phase shifters 25A and 25B.However, similarly to the first and second embodiments described above,the constitution may be such that the phase of one clock signal isadjusted by a phase shifter, to relatively control the phase differencebetween the two clock signals.

The output monitoring section 40′ branches a part of the CS-RZ signallight output from the latter modulator 20 as a monitor light by theoptical coupler 41, photo-electric converts the monitor light by thelight receiving circuit 42 to acquire the electric spectrum, anddirectly sends the electric spectrum to the electric power sensor 44without passing through an electric filter. The electric power sensor 44measures the intensity of the electric spectrum over the whole frequencyband (hereunder, total power), to output a signal indicating themeasurement result to the control circuit 50.

The control circuit 50 generates the control signal S_(FB) for feedbackcontrolling phase adjustment amounts of the respective phase shifters25A and 25B, so that the total power measured by the electric powersensor 44 becomes maximum. This feedback control by the control circuit50 is performed based on the characteristic of change in the electricspectrum of the CS-RZ signal light to a phase change between the clocksignal CLK1 and the clock signal CLK2 as described later.

FIG. 9 shows examples of the electric spectrum of the CS-RZ signal lightgenerated when a relative phase between the clock signals CLK1 and CLK2of 20 GHz is changed. Moreover, FIG. 10 shows examples of the opticalwaveform of the CS-RZ signal light generated when the phase is changedsimilarly to the case of FIG. 9. Here, with a condition where the phasebetween the clock signals CLK1 and CLK2 is optimized (phase shiftbetween the respective signals is 0°) as a reference, the phasedifference between the respective clock signals is changed until theoriginal optimum phase condition (phase shift is 360°) is restored afterthe phase shift continues to be gradually increased.

When the phase shift between the clock signals CLK1 and CLK2 isincreased, it is understood that, as shown in FIG. 10, the opticalwaveform of the output light is largely deteriorated and the outputlight is quenched when the phase shift reaches 180°. At this time, asshown in FIG. 9, it is apparent that the total power of the electricspectrum of the output light is decreased accompanying the increase ofthe phase shift between the clock signals CLK1 and CLK2.

Therefore, in the present embodiment, a generation condition of thephase shift is judged based on a change in the total power which ischanged corresponding to the above described phase shift between theclock signals CLK1 and CLK2, to feedback control the phase shifters 25Aand 25B so that the phase shift between the clock signals CLK1 and CLK2is optimized.

More specifically, in the case where the electric spectrum correspondingto the above described CS-RZ signal light of 40 Gb/s can be obtained bythe light receiving circuit 42, then as is also apparent from the changein the electric spectrum extracted in FIG. 11, the less the phase shiftbetween the clock signals CLK1 and CLK2 becomes, the more the totalpower measured by the electric power sensor 44 is increased. Therefore,the phase shifters 25A and 25B are feedback controlled so that the totalpower becomes a maximum. Thus, it becomes possible to optimize the phaseshift between the clock signals CLK1 and CLK2.

Furthermore, in the control circuit 50, the maximum value of theintensity measured by the electric power sensor 44 is detected byapplying the well-known processing such as the dithering. Thus, it isalso possible to detect a traveling direction of the phase shift. If inthis manner, the traveling direction of the phase shift is detected tofeedback control the phase shifters 25A and 25B, the phase differencebetween the clock signals CLK1 and CLK2 can be optimized at higherspeed.

As described above, according to the third embodiment, the total powerof the electric spectrum of the output light from the latter stage LNmodulator 20 is monitored. Therefore, the phase shift between the twoclock signals CLK1 and CLK2 driving the latter stage LN modulator 20 canbe reliably detected, to feedback control the phase shifters 25A and25B. As a result, it becomes possible to generate the CS-RZ signal lightin a stable drive condition.

In the above described second and third embodiments, the description hasbeen made on the optical modulator in which the former stage LNmodulator 10 is driven by the data signal of B (b/s) and the latterstage LN modulator 20 is driven by the clock signal of B/2 (Hz), togenerate the CS-RZ signal light of B (b/s). However, the presentinvention is not limited thereto, and similarly to the above describedcase exemplified in FIG. 2, the present invention can also be applied toan optical modulator in which the clock signal having the frequency ofB/2 (Hz) corresponding to the bit rate of the data signal driving theformer stage LN modulator 10 is given to one of the electrodes in thelatter stage LN modulator 20, to generate the RZ signal light of B(b/s).

Next is a description of a fourth embodiment of the control apparatusfor the optical modulator according to the present invention. In thefourth embodiment, the description is for the control apparatus capableof realizing a control corresponding to an operating point variation ofthe optical modulator.

First is a brief description of operating point variation of the opticalmodulator is to be controlled in the present embodiment. Generally, aMach-Zehnder optical modulator is used for generating the signal lightcorresponding to the CS-RZ modulation system. An advantage of thisoptical modulator is that a wavelength variation of the transmissionlight is small. However, there is a problem that, due to a temperaturechange or aging of the material used for the substrate (for example,lithium niobate), the operating point of an electro-optic conversioncharacteristic is varied with time.

In order to suppress this operating point variation, conventionallyconcerning the generation of signal light corresponding to the NRZmodulation system, a technique has been known, for example, for giving adrive signal superposed with a low frequency signal to a Mach-Zehnderoptical modulator, extracting a low frequency signal component containedin the output light to detect the operating point variation, and basedon the detection result, feedback controlling the DC bias of the opticalmodulator (for detail, refer to Japanese Unexamined Patent PublicationNo. 3-251815).

FIG. 12 is a block diagram showing a schematic configuration of thecontrol apparatus to which the above described well-known technique isapplied. Furthermore, FIG. 13 is a diagram for explaining the theory ofhow the control apparatus of FIG. 12 compensates for the operating point

In the constitution of FIG. 12, a low frequency signal (frequency isf0=1 kHz or the like) generated by an oscillator 227 is given to a drivecircuit 211 driving a Mach-Zehnder optical modulator 210, as a gaincontrol voltage, to generate an NRZ data signal which is amplitudemodified in accordance with the low frequency signal, as shown at thebottom left in FIG. 13, and the NRZ data signal is applied to anelectrode of the modulator 210. As a result, an input light from a lightsource 201 is externally modified. Then, after a part of the NRZ signallight output from the modulator 210 is branched as a monitor light by anoptical coupler 221, this branched light is converted into an electricsignal by an optical receiver (PD) 222, and the frequency f0 componentcontained in the electric signal is selectively amplified by anamplifier 223 to be sent to a phase comparator 224. In the phasecomparator 224, a comparison is performed between a phase of an outputsignal from the amplifier 223 and a phase of the low frequency signalfrom the oscillator 227, and a signal indicating the comparison resultis given, via a low-pass filter 225 eliminating unnecessary components,to a bias supply circuit 226, and the DC bias adjusting the operatingpoint of the modulator 210 is controlled.

An optimum operating point of the Mach-Zehnder optical modulator in theNRZ modulation, as shown by the curve “a” at the top left of FIG. 13, isa point where the high level and low level of the waveform of the drivesignal, the amplitude of which is set to Vπ, gives the maximum andminimum power of the output light. When the modulator 210 is driven atthis optimum operating point, then as shown at the top right of FIG. 13,the NZR signal light output from the modulator 210 does not contain thefrequency f0 component, but a component twice the frequency f0 componentis generated.

On the other hand, as shown by curves “b” and “c” at the top left ofFIG. 13, when the operating point of the modulator 210 is shifted fromthe optimum operating point, then according to a shifted direction,relative to an envelope of the high level or low level, the phases arereversed between the drive waveform and the output light waveform. Inthe output light at this time, as shown at the middle and bottom rightin FIG. 13, the envelopes of the high level and low level becomewaveforms modulated at the same phase, and contain the frequency f0component. The phase of the frequency f0 component contained in theoutput light is reversed when a variation direction of the operatingpoint is changed. Therefore, by comparing the phase of the frequency f0component with a phase of the low frequency signal superimposed on thedrive signal, it becomes possible to detect the variation direction ofthe operating point. Accordingly, by feedback controlling the DC biasapplied to the modulator 210 corresponding the phase comparison resultin the phase comparator 224, it becomes possible to drive the modulator210 at the optimum operating point.

In the case where the above compensation technique of the operatingpoint related to the generation of signal light corresponding to the NRZmodulation system is applied to the generation of signal light of theCS-RZ modulation system in which modulators of two stage configurationare used, then for one modulator performing the NRZ modulation based onthe data signal, the operating point can be effectively compensated.However, for the other modulator driven by the clock signal, the driveamplitude 2Vπ twice the NRZ modulation is used (refer to FIG. 27).Therefore, in the case where the operating point is deviated from theoptimum point, the envelopes of the output light waveform correspondingto the high level side and low level side of the low frequency modulateddrive signal, become opposite phases to counteract each other, and thefrequency f0 component cannot be detected from the output lightTherefore, differently from the CS-RZ modulation system or the RZmodulation system, the conventional operating point compensationtechnique as described above cannot be applied to the modulation systemwhich is driven between two peaks of emitted light or two peaks ofextinct light in the electro-optic conversion characteristic of theMach-Zehnder optical modulator.

Therefore, in the fourth embodiment of the present invention, forexample, for an optical modulator generating a signal lightcorresponding to the CS-RZ modulation system of B (b/s), there will bedescribed the control apparatus that has realized the operating pointcompensation of a modulator driven with the voltage amplitude of 2Vπusing a clock signal.

FIG. 14 is a block diagram showing the configuration of the controlapparatus for the optical modulator according to the fourth embodiment.

In FIG. 14, similarly to the first to the third embodiments, the controlapparatus of the fourth embodiment is applied to the optical modulatorin which the former stage LN modulator 10 is driven by the data signalDATA of B (b/s) and the latter stage LN modulator 20 is driven by theclock signals CLK1 and CLK2 of B/2 (Hz), to output the signal light ofthe CS-RZ modulation system. The constitution of the present embodimentdiffers from that of the other embodiments, in that there is provided abias supply circuit 27 giving the DC bias for adjusting the operatingpoint, to the latter stage LN modulator 20, and an operation of thisbias supply circuit 27 is feedback controlled by an output monitoringsection 60 and a control circuit 70, to compensate for the operatingpoint of the latter stage LN modulator 20.

Specifically, the output monitoring section 60 includes, for example, anoptical coupler 61, a light receiving circuit 62, an electric filter 63,and an electric power sensor 64. The optical coupler 61 is for branchinga part of the CS-RZ signal light output from the latter state LNmodulator 20 as a monitor light, to send the branched light to the lightreceiving circuit 62. The light receiving circuit 62 is forphotoelectric converting the monitor light branched by the opticalcoupler 61, to acquire the electric spectrum. The electric filter 63 isa narrow band electric band-pass filter capable of extracting afrequency component with the center frequency of B/2 (Hz), from theelectric spectrum obtained by the light receiving circuit 62. Theelectric power sensor 64 measures the intensity of the electric signalextracted by the electric filter 63 and outputs the measurement resultto the control circuit 70.

The control circuit 70 generates the control signal S_(FB) for feedbackcontrolling the setting of the operation of the bias supply circuit 27so that the intensity of the frequency component with the centerfrequency of B/2 (Hz) measured by the electric power sensor 64 becomes aminimum.

Here, although not shown in the figure, the operating point deviation ofthe former stage LN modulator 10 is compensated for by applying theconventional compensation system in which the above described lowfrequency signal is superimposed on the drive signal, to compensate forthe operating point.

Next is a specific description of the theory of operating pointcompensation for the latter stage LN modulator 20 in the presentembodiment.

FIG. 15 is a diagram for explaining a change in output light when theoperating point deviation occurs in the modulator driven by the voltageamplitude of 2Vπ.

For example, when the clock signals CLK1 and CLK2 of 20 GHz are given tothe respective electrodes 20A and 20B of the latter stage LN modulator20, as shown at the left in FIG. 15, the potential difference betweenthe respective electrodes 20A and 20B is changed at the amplitude of 2Vπcorresponding to one period of the periodic optical intensitycharacteristic of the LN modulator 20. At this time, if the operatingpoint of the LN modulator 20 is deviated from the optimum point, it isunderstood that, in the waveform of the CS-RZ signal light of 40 Gb/soutput from LN modulator 20, as shown at the top right in FIG. 15, thedeviation occurs in the levels between the respective bits. Moreover, inthe electric spectrum of the output light, as shown at the middle rightin FIG. 15, it is understood that a peak is generated at the frequencyof 20 GHz, which was not seen in the case where the operating point isset in optimum (for example, refer to FIG. 5 and FIG. 9). Furthermore,in the optical spectrum of the output light, as shown at the bottomright in FIG. 15, it is understood that a carrier componentcorresponding to the central optical frequency is generated, which wasnot seen in the case where the operating point is set in optimum (forexample, refer to the bottom right in FIG. 27).

Considering the abovementioned change characteristic of the output lightfor when the operating point deviation occurs, in the presentembodiment, the intensity of the frequency component with the center of20 GHz of the electric spectrum, that is, the frequency component withthe center of B/2 (Hz) corresponding to the frequency of the clocksignals CLK1 and CLK2 driving the LN modulator 20, is monitored. Basedon the monitoring result, the occurrence condition of the operatingpoint deviation of the LN modulator 20 is judged, and the DC bias isfeedback controlled so that the operating point is optimized.

Specifically, in the case where the electric spectrum corresponding tothe CS-RZ signal light of 40 Gb/s exemplified in FIG. 15 can be obtainedby the light receiving circuit 62, then as shown by the broken lineportion in FIG. 16, the central frequency of a transmission band of theelectric filter 63 is set, to match with the 20 GHz being the frequencyof the clock signals CLK1 and CLK2. As is also apparent from FIG. 16,the closer the operating point comes to the optimum point, the more theintensity of the frequency component extracted by this electric filter63 is decreased. Therefore, the setting of the operation of the biassupply circuit 27 is feedback controlled so that the intensity measuredby the electric power sensor 64 becomes a minimum. Thus, it becomespossible to optimize the operating point of the LN modulator 20.

Furthermore, in the control circuit 70, it is also possible to detect aminimum value of the intensity measured by the electric power sensor 64by applying a well-known processing such as the dithering, to therebydetect the traveling direction of the operating point deviation. If inthis manner, the traveling direction of the operating point deviation isdetected and the DC bias is feedback controlled, the operating point ofthe LN modulator 20 can be compensated at higher speed.

As described above, according to the fourth embodiment, for the LNmodulator 20 driven by the clock signals CLK1 and CLK2 of B/2 (Hz), theintensity change in the frequency component with the center of B/2 (Hz)of the electric spectrum of the output light is monitored. Therefore,the generation condition of the operating point deviation can bereliably detected, to feedback control the DC bias. As a result, itbecomes possible to realize the operating point compensation for themodulator driven with the voltage amplitude of 2Vπ, which has beendifficult in the conventional system in which the low frequency signalis superimposed on the drive signal for compensating for the operatingpoint. Thus, it becomes possible to generate the CS-RZ signal light in astable drive condition.

Next is a description of a fifth embodiment of the control apparatus forthe optical modulator according to the present invention.

In the above described fourth embodiment, the description has been madeon the case where the operating point is compensated, paying attentionto the change in the electric spectrum in the change characteristic ofthe output light due to the operating point deviation shown in FIG. 15.In the fifth embodiment, the description is for the case where theoperating point is compensated, paying attention to a change in theoptical spectrum in the change characteristic of the output light.

FIG. 17 is a block diagram showing a configuration of the controlapparatus for the optical modulator according to the fifth embodiment

In FIG. 17, the constitution of the present embodiment differs from thatof the fourth embodiment shown in FIG. 14 described above, in that,instead of the output monitoring section 60 and the control circuit 70,there are provided: an output monitoring section 80 that branches a partof the signal light output from the latter stage LN modulator 20 as amonitor light, extracts the central optical frequency component, andmonitors the optical power of the central optical frequency component;and a control circuit 90 judging the operating point deviation of the LNmodulator 20, to feedback controls the bias supply circuit 27, based ona change in the optical power of the central optical frequency componentmonitored by the output monitoring section 80. Other components aresimilar to those of the fourth embodiment

The output monitoring section 80 includes, for example, an opticalcoupler 81, a narrow band optical filter 82, and an optical power meter83. The optical coupler 81 branches a part of the CS-RZ signal lightoutput from the latter stage LN modulator 20 as a monitor light, to sendthe branched light to the narrow band optical filter 82. The narrow bandoptical filter 82 has a filter characteristic in which the bandwidth ofthe transmission band is sufficiently narrow, and extracts only thecentral optical frequency component from the monitor light branched bythe optical coupler 81. The optical power meter 83 measures the power ofthe monitor light extracted by the narrow band optical filter 82 andoutputs the measurement result to the control circuit 90.

The control circuit 90 feedback controls the setting of the operation ofthe bias supply circuit 27 so that the power of the monitor lightmeasured by the optical power meter 83 becomes a minimum. This feedbackcontrol is performed based on a change in a carrier componentcorresponding to the central optical frequency, which is generated inthe output optical spectrum due to the operating point deviation asshown in FIG. 15. This carrier component of the central opticalfrequency is generated as a result that the symmetry property of thedual system clock signals CLK1 and CLK2 given to the respectiveelectrodes 20A and 20B of the LN modulator 20 is fractured and thecarrier suppression is not performed.

Specifically, in the case where a part of the CS-RZ signal light of 40Gb/s having the optical spectrum exemplified at the bottom right in FIG.15 is branched as a monitor light by the optical coupler 81, then asshown by the broken line portion in FIG. 18, the central opticalfrequency of the transmission band of the narrow band optical filter 82is set to match with the central optical frequency fc of the opticalspectrum of the CS-RZ signal light. As is also apparent from FIG. 18,the closer the operating point comes to the optimum point, the more theoptical power of the optical frequency component extracted by thisnarrow band optical filter 82 is decreased. Therefore, the setting ofthe operation of the bias supply circuit 27 is feedback controlled sothat the optical power measured by the optical power meter 83 becomes aminimum. Thus, it becomes possible to optimize the operating point ofthe LN modulator 20.

Furthermore, in the control circuit 90, a minimum value of the opticalpower measured by the optical power meter 83 is detected by applying awell-known processing such as the dithering. Thus, it is possible todetect the traveling direction of the operating point deviation. If inthis manner, the traveling direction of the operating point deviation isdetected, to feedback control the DC bias, the operating point of the LNmodulator 20 can be compensated at higher speed.

As described above, according to the fifth embodiment, for the LNmodulator 20 driven by the clock signals CLK1 and CLK2 of B/2 (Hz), thechange in the optical power of the carrier component generated at thecentral optical frequency of the output optical spectrum is alsomonitored. Therefore, the occurrence condition of the operating pointdeviation can be reliably detected, to feedback control the DC bias. Asa result, the operating point compensation for the modulator driven withthe voltage amplitude of 2Vπ can be realized. Thus, it becomes possibleto generate the CS-RZ signal light in a stable drive condition.

In the above described fifth embodiment, the configuration example hasbeen shown in which the optical power of the central optical frequencycomponent extracted by the narrow band optical filter 82, is measured bythe optical power meter 83. However, for example as shown in FIG. 19, itis also possible to provide a light receiving element 84 and an electricpower sensor 85 instead of the optical power meter 83, to measure theoptical power of the central optical frequency component

Furthermore, in the above described first through fifth embodiments, anexample has been shown in which the individual components are connectedin series as the former and latter stage LN modulators 10 and 20.However, for example, the former and latter stage LN modulators 10 and20 may be continuously formed on the same substrate, for example.

Moreover, for the above described first through fifth embodiments, it isalso possible to appropriately combine the respective components, toconcurrently perform any two or more compensations from among: thecompensation of the phase shift between the data signal driving theformer stage LN modulator 10 and the clock signals driving the latterstage LN modulator 20: the compensation of the phase shift between thedual system clock signals driving the latter stage LN modulator 20, andthe compensation of the operating point of the latter stage LN modulator20. Hereunder are specific embodiments related to the above describedcombinations.

FIG. 20 is a block diagram showing an embodiment of the controlapparatus for the optical modulator, in which the configurations of thefirst, third and fourth embodiments are combined. In the configurationof this embodiment, the following compensations are concurrentlyperformed, namely: the compensation of the phase shift between the datasignal and the clock signals realized by the feedback control of thephase shifter 23 by the phase comparator 30 and the control circuit 31;the compensation of the phase shift between the clock signals CLK1 andCLK2 realized by the feedback control of the phase shifter 25 by theoutput monitoring section 40′ and the control circuit 50; and thecompensation of the operating point of the latter stage LN modulator 20realized by the feedback control of the bias supply circuit 27 based onthe electric spectrum of the output light, by the output monitoringsection 60 and the control circuit 70.

Furthermore, for the configuration of the embodiment shown in FIG. 20,it is also possible to simplify the configuration, for example as shownin FIG. 21, by making common the optical coupler 41, the light receivingcircuit 42, and the electric power sensor 44 in the output monitoringsection 40′, and the optical coupler 61, the light receiving circuit 62,and the electric power sensor 64 in the output monitoring section 60,and providing a control CPU 91 equipped with the functions of both thecontrol circuits 50 and 70.

FIG. 22 is a block diagram showing an embodiment of the controlapparatus for the optical modulator, in which the configurations of thefirst, third and fifth embodiments are combined. In the configuration ofthis embodiment, the following compensations are concurrently performed,namely: the compensation of the phase shift between the data signal andthe clock signals realized by the feedback control of the phase shifter23 by the phase comparator 30 and the control circuit 31; thecompensation of the phase shift between the clock signals CLK1 and CLK2realized by the feedback control of the phase shifter 25 by the outputmonitoring section 40′ and the control circuit 50; and the compensationof the operating point of the latter stage LN modulator 20 realized bythe feedback control of the bias supply circuit 27 based on the opticalspectrum of the output light, by the output monitoring section 80 andthe control circuit 90.

FIG. 23 is a block diagram showing an embodiment of the controlapparatus for the optical modulator, in which the configurations of thesecond, third and fourth embodiments are combined. In this combination,since the respective compensations are performed based on the electricspectrum of the output light, it is possible to simplify theconfiguration by making common the optical coupler, the opticalreceiving circuit, and the electric power sensor in the outputmonitoring section corresponding to each of the compensations, so that acontrol CPU 92 feedback controls the phase shifters 23 and 25, and thebias supply circuit 27, respectively, based on the measurement result bythe electric power sensor.

FIG. 24 is a block diagram showing an embodiment of the controlapparatus for the optical modulator, in which the configurations of thesecond, third and fifth embodiments are combined. In the configurationof this embodiment, the change in the electric spectrum of the outputlight is monitored by the output monitoring section in which the opticalcoupler, the optical receiving circuit, and the electric power sensorare made common, and a control CPU 93 feedback controls the phaseshifters 23 and 25, respectively, based on the monitoring result. As aresult, the phase shift between the data signal and the clock signals,and the phase shift between the clock signals CLK1 and CLK2 arecompensated. Moreover, at the same time, by the feedback control of thebias supply circuit 27 based on the optical spectrum of the output lightby the output monitoring section 80 and the control circuit 90, theoperating point of the latter stage LN modulator 20 is compensated.

The above described configurations shown in FIG. 20 through FIG. 24 showpreferable specific examples of combinations of the above describedfirst through fifth embodiments. Similarly to these, it is of coursepossible to configure a control apparatus for an optical modulator byother combinations.

1. A control apparatus for an optical modulator that comprises: anoptical modulation section including a part for branching an opticalwaveguide into a first branch optical waveguide and a second branchoptical waveguide, and a part for combining said first and second branchoptical waveguides, and utilizing a first electrode and a secondelectrode respectively provided in said first and second branch opticalwaveguides to control refractive indexes of said first and second branchoptical waveguides, and obtaining a periodic optical intensitycharacteristic according to a difference between the refractive indexes;a drive section that gives a drive signal to at least one of said firstand second electrodes so that said optical modulation section performs amodulating operation corresponding to one period of the opticalintensity characteristic thereof; and a bias supply section thatsupplies a DC bias to said optical modulation section to adjust anoperating point, wherein said control apparatus comprises: an outputmonitoring section that detects a change in the signal light output fromsaid optical modulation section, wherein said output monitoring sectionacquires the optical spectrum of a signal light output from said opticalmodulation section, and extracts from said optical spectrum an opticalfrequency component matched to the central optical frequency of saidsignal light, to detect an optical power of said optical frequencycomponent; and a control section that judges the operating pointdeviation of said optical modulation section according to the opticalpower detected by said output monitoring section, and controls said biassupply section so as to minimize said operating point deviation.
 2. Acontrol method for an optical modulator that comprises: an opticalmodulation section including a part for branching an optical waveguideinto a first branch optical waveguide and a second branch opticalwaveguide, and a part for combining said first and second branch opticalwaveguides, and utilizing a first electrode and a second electroderespectively provided in said first and second branch optical waveguidesto control refractive indexes of said first and second branch opticalwaveguides, and obtaining a periodic optical intensity characteristicaccording to a difference between the refractive indexes; a drivesection that gives a drive signal to at least one of said first andsecond electrodes so that said optical modulation section performs amodulating operation corresponding to one period of the opticalintensity characteristic thereof; and a bias supply section thatsupplies a DC bias to said optical modulation section to adjust anoperating point, wherein said control method comprises: detecting achange in the signal light output from said optical modulation section,by acquiring the optical spectrum of a signal light output from saidoptical modulation section, and extracting from said optical spectrum anoptical frequency component matched to the central optical frequency ofsaid signal light, to detect an optical power of said optical frequencycomponent; judging the operating point deviation of said opticalmodulation section according to the detected optical power of saidoptical frequency component; and controlling said bias supply section soas to minimize said operating point deviation.
 3. A control apparatusfor an optical modulator that comprises: an optical modulation sectionincluding a branched optical waveguide; a drive section that provides adrive signal to the optical modulation section; and a bias supplysection that supplies a bias to the optical modulation section to adjustan operating point, wherein said control apparatus comprises: an outputmonitoring section that detects a change in the signal light output fromthe optical modulation section, wherein the output monitoring sectionacquires the optical spectrum of a signal light output from the opticalmodulation section, and extracts from the optical spectrum an opticalfrequency component matched to the central optical frequency of thesignal light, to detect an optical power of the optical frequencycomponent; and a control section that judges the operating pointdeviation of the optical modulation section according to the opticalpower detected by the output monitoring section, and controls the biassupply section so as to minimize the operating point deviation.